Carbon dioxide is a commonly occurring component of gas and oil streams. Carbon dioxide is also used in secondary oil recovery as a miscible flood component. Wellbores and wells must therefore be provided that can tolerate the presence of carbon dioxide for use as either injection wells or production wells. A problem in providing such an injection or production well is that Portland cements are typically used to cement casings in such injection and production wells, and Portland type cements are susceptible to acid attack by dissolved carbon dioxide which forms carbonic acid. Portland cements contain calcium hydroxide, and calcium hydroxide reacts with carbonic acid. Particularly at elevated temperatures, continual influx of carbonic acid convert Portland cement to a soft amorphous gel through a variety of reactions. Portland cements therefore gradually degrade while exposed to carbon dioxide. The inherently high permeability of Portland cements also causes a large portion of the cement to be exposed to this carbonic acid attack.
Eventually, remedial squeeze cementing is required to repair degraded Portland cements, or the cement will fail. This is very undesirable due to the high cost of squeeze cementing, and the unlikelihood that a squeeze cement will result in the quality cement job that is required.
Cementing is particularly difficult when carbonic acid is present when the cement is to be set. Carbonic acid inhibits proper setting of Portland type cements. Expensive additives are therefore added to the cement slurry to enable the Portland cement slurries to set when the slurry must cure in the presence of carbonic acid. Permeability of Portland cements can also be reduced by inclusion of additives in the cement slurry and the use of higher density slurries, but these measures also increase the cost of the cement.
It would therefore be desirable to provide a method to cement wellbores in the presence of carbon dioxide or carbonic acid and a method to cement wellbores when the wellbores will subsequently be contacted with carbon dioxide or carbonic acid wherein the cement is not degraded by carbon dioxide or carbonic acid to the extent that Portland cements are degraded by carbon dioxide or carbonic acid.
It is therefore an object of the present invention to provide a method to cement wellbores in the presence of carbon dioxide or carbonic acid and where the wellbore will be exposed to carbon dioxide or carbonic acid after the cement cures wherein the cement has improved tolerance to carbon dioxide or carbonic acid and wherein the cement will cure in the presence of carbon dioxide or carbonic acid.